1. Field of the Invention
This invention relates to the field of materials science and more particularly to the growth of semiconductor crystals.
2. Description of the Related Art
There is currently a demand in the computer and telecommunication industries for multicolor light emitting displays and improved data density in communication and recording. As a result of this demand, there is a strong desire for a semiconductor light emitting element capable of emitting light having shorter wavelengths ranging from a blue light wavelength to an ultraviolet wavelength.
The III-V nitrides, as a consequence of their electronic and optical properties and heterostructure character, are highly advantageous for use in the fabrication of a wide range of microelectronic structures. In addition to their wide band gaps, the III-V nitrides also have direct band gaps and are able to form alloys which permit fabrication of well lattice-matched heterostructures. Consequently, devices made from the III-V nitrides can operate at high temperatures, with high power capabilities, and can efficiently emit light in the blue and ultraviolet regions of the electromagnetic spectrum. Devices fabricated from III-V nitrides have applications in full color displays, super-luminescent light-emitting diodes (LEDs). high density optical storage systems. and excitation sources for spectroscopic analysis applications. Furthermore, high temperature applications are found in automotive and aeronautical electronics.
Effective use of these advantages of the III-V nitrides, however, requires that such materials have device quality and structure accommodating abrupt heterostructure interfaces. As such, the III-V nitrides must be of single crystal character and substantially free of defects that are electrically or optically active.
Gallium nitride (or GaN) is a particularly advantageous III-V nitride and attention has recently focused on gallium nitride related compound semiconductors (In(x)Ga(y)Al(1-x-yN) (0xe2x89xa6x, y; x+yxe2x89xa61) as materials for emitting blue light. This nitride species can be used to provide optically efficient, high temperature, wide band gap heterostructure semiconductor systems having a convenient, closely matched heterostructure character. The direct transition type band structure of GaN permits highly efficient emission of light. Moreover, GaN emits light of shorter wavelength ranging from a blue light wavelength to an ultraviolet wavelength, due to a great band gap at room temperature of about 3.4 eV.
As no GaN substrates are currently found in the art, growth of these compounds must initially take place heteroepitaxially, for example GaN on silicon. However, heteroepitaxial growth has several technical drawbacks. In particular, two types of defects arise as a result of heteroepitaxial growth: (i) dislocation defects due to lattice mismatch: and (ii) dislocation defects due to different thermal coefficients between the substrate and the epitaxial layer.
The first type of defect includes dislocations due to the lattice mismatch between the GaN layer and the substrate. One typical substrate is sapphire. In the case where a gallium nitride related compound semiconductor crystal is grown on a sapphire substrate, a lattice mismatch up to approximately 16% is found between the GaN and the substrate. SiC is a closer lattice match, at an approximate lattice mismatch of 3%, but the mismatch is still quite large. Many other substrates have been used, but all of them have large lattice mismatches and result in a high density of defects in the grown layers.
The second type of defect includes dislocations generated during cool-down after growth. This defect is a result of different thermal coefficients of expansion of the substrate and epitaxial layer.
There are two typical methods in use for growing GaN compound semiconductor crystals. However, both suffer from deficiencies and/or limitations adversely affecting the quality of the GaN product. A first method uses a single crystalline sapphire as a substrate. A buffer layer is grown on the substrate for the purpose of relaxation of lattice mismatching between the sapphire substrate and the GaN compound semiconductor crystal. The buffer layer may be a AIN buffer layer or a GaAlN buffer layer. A GaN compound semiconductor crystal is grown in the buffer layer. While the buffer layers improve the crystallinity and surface morphology of the GaN compound semiconductor crystal, the crystal remains in a distorted state because of the lattice mismatch with the sapphire substrate. This distorted state results in dislocation defects described herein.
A second method attempts to reduce the lattice mismatch by providing a single crystal material as a substrate having a crystal structure and lattice constant that closely matches that of the GaN compound semiconductor crystal. One embodiment of this method uses aluminum garnet or gallium garnet as a substrate, but the lattice match using these compounds is not sufficient to provide much improvement. Another embodiment of this method uses substrate materials including MnO, ZnO, MgO, and CaO. While these oxides provide a better lattice match with the substrate, the oxides undergo thermal decomposition at the high temperatures required for the growth of the GaN compound semiconductor. Thermal decomposition of the substrate adversely affects the quality of the semiconductors obtained using this method.
As a result of these problems, typical GaN semiconductor devices suffer from poor device characteristics, short life span, and high cost. Full utilization of the properties of GaN semiconductors cannot be realized until a suitable substrate is available that allows for growth of high quality homoepitaxial layers. This requires development of processes for growth of the substrate material. For device applications, therefore, it would be highly advantageous to provide substrates of GaN, for epitaxial growth thereon of a GaN crystal layer. Thus, it would be a significant advance in materials science to provide GaN in bulk single crystal form, suitable for use as a substrate body for the fabrication of microelectronic structures.
A method and apparatus for homoepitaxial growth of freestanding, single bulk crystal Gallium Nitride (GaN) are provided. The fabrication method includes a step of nucleating GaN in a reactor at a temperature less than approximately 800 degrees Celsius and a pressure substantially in the range of 10xe2x88x923 Torr to 10xe2x88x926 Torr. This nucleation phase results in a first GaN structure, or GaN nucleation layer, having a thickness of a few monolayers. The nucleation layer is stabilized, and a single bulk crystal GaN is grown from gas phase reactants on the GaN nucleation layer in the reactor at a temperature substantially in the range of 450 degrees Celsius to 1250 degrees Celsius and a pressure substantially in the range of 10xe2x88x923 Torr to atmosphere. The reactor is formed from ultra low oxygen stainless steel.
The descriptions provided herein are exemplary and explanatory and are provided as examples of the claimed invention.